Reptiles are vertebrates (have a backbone), and share many anatomical features with birds and mammals. However, reptiles are different from birds and mammals in how their bodies operate and create heat. Birds and mammals are endotherms; they mainly create body heat metabolically (using energy from food; endo means ‘from within’). Birds and mammals can maintain a high and constant body temperature even in cool conditions. This means that some birds and mammals can live in cold places and be active year-round.

In comparison, reptiles are ectotherms; they generate almost no body heat metabolically. Instead, reptiles use heat from the environment, such as heat from sunlight or warm soil (ecto means ‘from outside’). While some large reptiles such as pythons and marine turtles heat themselves by shivering (like you do on a cold day), most still need environmental supplies of heat to provide the energy to move and keep their body working.

Reptiles need temperatures that are not too low and not too high. Just like you, most reptiles like to keep their body at a certain temperature. Because they can’t change their temperature by metabolism alone, instead, they change their behaviour! Reptiles maintain their body temperature by moving from place to place, putting part of their body in the shade and part in the sun, or changing how much of their body is touching the ground. The range of environmental temperatures over which they can do this is fairly limited. In cool conditions, reptiles also become cool and very sluggish. Where it is too cold for too long, reptiles simply cannot live.

Ectothermy seems like a difficult life in comparison with endothermy. Endothermy certainly is beneficial to birds and mammals, allowing them to live in environments (e.g. northern climates and high elevations) that ectotherms cannot enjoy. However, endothermy comes with a price: in order to maintain high and constant body temperatures, endotherms have a much higher metabolic rate than ectotherms. That high metabolic rate is fuelled by frequent feeding.

Reptiles, because they are ectotherms, do not eat as much or as often as endotherms of the same body size. When food is scarce, this is an advantage: ectotherms are much better at surviving food shortages than endotherms because they need less energy. For the same reason, aquatic reptiles can stay under water for long periods without coming up for air. Ectotherms also can allow their body temperatures to fluctuate up and down more than endotherms can. True, some birds and mammals can lower their body temperatures during winter hibernation or nightly torpor (state of low metabolic activity). However, ectotherms can allow their body temperatures to fluctuate to a much greater degree without hurting themselves, even when they are active! When food or water is short, reptiles can hide in cool places, lowering their body temperature and metabolic rate, and conserving food stores and energy.

Despite the advantages of ectothermy in some situations, cold does limit the number and types of reptile species found in the north. Most ectotherms are not adapted to survive the very cold winters in Canada. In British Columbia, almost all of our native reptile species are limited to the extreme south of the province. Only three species of snakes are able to live very far north.

In northern British Columbia, winter weather is too cold for reptiles to be active. If they stayed in the open, they would freeze to death. To live in the north, reptiles have to find some way to avoid exposure to cold in winter.

Many birds escape the cold winters by migrating to warmer places, but, aside from marine turtles, reptiles can’t travel large distances. Instead, reptiles must either tolerate the cold or go underground or underwater to escape it. One way of tolerating the cold is to ‘supercool’ (lower the body temperature below 0oC without freezing body fluids – not something you should try at home) or to tolerate freezing directly (survive freezing of the body).

Some northern amphibians can survive freezing of a large part of their bodies, but this seems to be a less common strategy in reptiles. There are a few exceptions. Hatchling painted turtles appear to survive winter through a combination of supercooling and freeze tolerance. Garter snakes also seem to have some tolerance for freezing. Most of our reptiles though, including garter snakes, avoid winter weather (and freezing temperatures) by hibernating. Except for hatchlings, painted turtles do this by going to the bottom of ponds or lakes that do not freeze solid. Even though the surface of the lake may be covered in ice, hibernating painted turtles survive by absorbing the small amount of oxygen they need from the water. In comparison, lizards and snakes hibernate on land, using burrows and cavities to get underground and below the frost line (the depth that the surface freezes to).

Common garter snake

In places where winters are very cold and the frost line is very deep, sites where reptiles can get deep enough underground to survive the winter may be rare. In these cases, hibernating sites may be shared by many animals, and animals may travel from far away to use the site. Although such communal hibernation also is sometimes seen in mild climates, such as that of Vancouver Island, it is more common where winters are cold.

Rattlesnake

This behaviour also appears to be more common in certain species. In British Columbia, garter snakes and rattlesnakes have been found in large communal hibernacula. Because communal hibernation sites may be few and far between, snakes that use them often have to migrate relatively long distances to and from summer habitats.

Although winter is a challenge, northern reptiles also are limited by short, relatively cool summers. Cool, short summers mean that the opportunities to feed, grow and reproduce are limited. Reproduction may be an especially critical issue. Worldwide, most reptile species are oviparous (laying eggs), but some lizards and snakes are viviparous (bearing live young). However, the proportion of viviparous species is high in cold climates. In British Columbia, for example, of 11 native species of lizards and snakes, six are viviparous and it is only viviparous species that live in northern B.C.

This is a complex issue. The simple answer has to do with temperature and developmental rates of embryos. Embryos develop faster at higher temperatures; if they are too cold, they may develop too slowly to be born before winter, or they may even die. Oviparous, or egg-laying species, lay and leave their eggs in a nest. They have little control over the fate of the eggs, and eggs are vulnerable not only to predation, but also to mortality due to cold weather.

In comparison, females of viviparous species can maintain, through their own behavioural thermoregulation (regulation of body temperature), a better temperature for the development of their young. In fact, many experiments have shown that pregnant females of live-bearing species maintain a more precise body temperature than do snakes that are not pregnant.

Egg-laying species keep their eggs inside their bodies for some time before laying them and can achieve some of the same benefits as live-bearing species, but not for the entire period of embryo development. Thermoregulatory behaviour during egg retention may be important for the reproductive success of those few oviparous species that occur in northern regions elsewhere. Viviparity may have other advantages over oviparity in other situations, including in warmer climates; for example, in tropical sea snakes, viviparity allows females to avoid ever coming ashore, where they move awkwardly and would be exposed to predators.

Just as there are tradeoffs between endothermy and ectothermy, both viviparity and oviparity have advantages and disadvantages. Above, we focused on the advantages of viviparity and the disadvantages of oviparity in northern environments. However, remember that a pregnant female of a viviparous species is burdened with the weight of her offspring for much of the summer. This limits her ability to escape from predators, so she cannot move too far from shelter. It also limits her ability to capture active prey, which may not be abundant in places that are good for pregnant females. Females usually reduce the amount of food they eat during pregnancy and may be very skinny after they give birth. They then have to make up the weight lost by feeding before winter. At a minimum, they need to eat enough to be able to survive the winter, but they will need to eat even more if they are to be fat enough to reproduce again the next year.

In fact, females of viviparous species often skip reproduction in some years. Female rattlesnakes in British Columbia never reproduce in consecutive years and skip reproduction for anywhere from 1-4 years. Females of oviparous species should have longer periods for feeding and have to skip fewer reproductive years. However, oviparous species in places like B.C. have been studied less, and more research is needed.